The field of the invention is data acquisition systems, and particularly, systems for polling remote stations and inputting data from them.
Numerous data acquisition systems are employed in industrial applications. Such systems typically employ a master controller which is often a programmed digital computer, and one or more remote sensing devices, or stations. The master controller "polls", or "addresses", the remote stations and they respond by transmitting data to the master controller. Such data may indicate the state of various sensing devices, such as limit switches, pressure gauges and thermometers. Such systems uniformly communicate information as digital signals in which logic states are represented by specific voltage or current levels.
In certain industrial environments extraordinary measures must be taken to prevent combustion of volatile gases. Where electrical signals must be conveyed from sensing devises located in such an environment, it is common practice to employ expensive conduits and so-called "explosion proof" enclosures for the electrical devices and wiring. More recently, efforts have been made to employ electrical devices which require low-level signals for operation. Such devices and the resulting electrical system are referred to in the art as "intrinsically safe", since they cannot produce enough power to initiate combustion and they comply with Underwriters Laboratories Standard 913.